Changes in molecular dynamics during the bulk polymerisation of an epoxide/boroxine mixture as studied by dielectric relaxation spectroscopy, revealing direct evidence for a floor temperature for reaction

Real-time dielectric relaxation spectroscopy (DRS), has been used to monitor changes in the dynamics of chain dipoles and of ions during the bulk polymerisation of an epoxide/boroxine system at different reaction temperatures TR in the range 60–120°C. For TR<72°C the dielectric α-process moved to ultra-low frequencies indicating the formation of a chemically unstable glass. For TR>72°C the process moved from high frequencies to settle at audio or higher frequencies, giving a loss peak independent of time that indicated the formation of a chemically stable elastomer. We analysed our results in terms of changes in molecular mobility in the reaction mixture with time and have determined the floor-temperature TF below which a glass is formed and above which an elastomer is formed. The DRS data were also presented as complex impedance Z(ω) which emphasises changes of ionic conductance during reaction. Below TF the peak in the loss impedance Z″m moved to ultra-low frequencies during reaction in a time-scale far shorter than that for the dielectric loss peak ε″m. Comparison of the two representations of data made well above TF show that the Z″ data become independent of time before the ε″ data. DRS data presented as dielectric permittivity ε(ω) provides a good indicator of glass-formation below TF and of elastomer-formation above TF. Our data are considered in relation to the TTT-diagram of Gillham and coworkers that describes the onset of gelation, glass-formation and elastomer-formation during cure and we extend this diagram to describe how Tg evolves with time for different values of TR.